The investigation into the crash of a Bristow Helicopters Sikorsky S-76A into the North Sea last month has revealed that a main rotor blade manufacturing anomaly, combined with the thermal damage from an earlier lightning strike, led to the catastrophic failure of the blade spar and the loss of the helicopter with all 11 crew and passengers.
A metallurgical examination by Qinetiq found that the tang tip - around 9.5 x 4.8mm (3/8in x 3/16in) long - on the scarf joint at the inboard end of the outboard erosion strip was bent and folded under the outboard end of the inboard erosion strip. The fatigue fracture was traced to the rear of the doubled-up erosion strip, where "intense thermal damage" from a 1999 lightning strike was also found.
The UK Air Accidents Investigation Branch (AAIB) says it and Sikorsky believe "that the electrical energy imparted by the lightning strike in 1999 exploited the anomaly that was built into the blade at manufacture and damaged the spar".
The anomaly and thermal damage to the spar appear to have gone unnoticed when the blade was sent to Sikorsky for inspection and repair after the lightning strike.
Sikorsky has moved to a one- piece leading edge in the 21 years since the failed blade was produced, but the concern is that there may be other blades with similar manufacturing anomalies.
The USFederal Aviation Administration has ordered an inspection of blades with a two-piece leading-edge erosion strip and the removal and inspection of any blades with a history of lightning strikes.
"We are also working with our customers to develop a process to reconfirm the condition of leading-edge blade sheaths," says Sikorsky, which is changing its S-76 maintenance manual to include periodic inspections.
Source: Flight International
